Barrier-type anodic films 3-15 nm thick have been formed on electropolished 99.999% aluminium. Variable-angle XPS has been used to identify a significant proportion of hydroxyl ions at the surface of the relatively compact alumina films. The location of an oxygen-rich region at the outer surface of the oxide has been confirmed by medium-energy ion scattering (MEIS). Combining the information from these two techniques leads to the conclusion that a hydroxyl-containing surface region is responsible for this oxygen-rich surface layer, MEIS revealing an approximately linear relationship between the total oxide thickness and the thickness of the hydroxyl-rich surface region. From consideration of the mechanisms of amorphous alumina formation by ionic transport, with incorporation of electrolyte-derived species into the thickening film, the generation of the hydroxyl-rich outermost region is considered to result from the formation of gel-like material at the film/electrolyte interface.